Steel laminations typically employed for motors, transformers, and other electrical devices are annealed in continuous roller hearth furnaces to improve magnetic properties. Steel laminations form the magnetic path in electrical devices which allows transfer of magnetic flux such as from a stator to a rotor in the case of a motor, for example, or completing a magnetic circuit in the case of a transformer. The performance of the laminations depends on the magnetic properties of the steel used to make the laminations. Typically the best magnetic properties are achieved following heat treatment or annealing of the laminations after stamping.
Annealing is a process of heat treatment of steel to both soften and to impart specific magnetic properties.
High productivity processes are important for annealing, and therefore continuous roller hearth furnaces are employed in the prior art. The rollers in the furnace are driven externally so that trays of laminations progress continuously through the furnace and the different processes involved.
A typical prior art lamination furnace comprises a high heat section, a controlled cooling section, a purge or transfer vestibule, a controlled oxidation section, and an air cool section.
It is known to provide the high heat section of the furnace with a controlled atmosphere that prevents the laminations from oxidizing during heating, and allows decarburization or removal of carbon.
Temperatures in the laminations are increased from approximately 650° F. to a soak temperature in the range of 1450-1600° F. depending upon the alloy content of the steel. The time of transfer through the high heat section can range from 90 to 150 minutes, depending on the carbon content and alloy content of the steel. An important critical part of the heating cycle is the actual soak time at the specified temperature, which is a function of both alloy content and carbon content. From a fundamental perspective, the soak time at temperature is fixed for each grade to permit both decarburization and recrystallization.
The controlled cooling section of the furnace allows the rows of laminations to cool uniformly from the soak temperatures of the high heat section to a temperature at which a uniform blue/gray oxide may be formed in the oxidation section. In most cases, the atmosphere is similar for the heating and cooling sections of the furnace and the sections are not physically separated. Although the exact cooling rate in the cooling section is not important for silicon alloy steels, it is important that the laminations achieve a uniform surface temperature throughout the load that, on exit from the purge sections, does not exceed 1000° F. An unstable iron oxide forms above this temperature and breaks down into cosmetically deleterious iron oxide products. Below 600° F., cosmetic problems also occur. Non-uniformity of loading or the inclusion of rows of laminations of different sizes leads to non-uniformity of cooling. In most cases this will result in non-uniform cosmetics since different oxide compositions and thicknesses form at different temperatures. Thus, control of uniformity of cooling is important.
It is also known to provide a purge or transfer vestibule to separate the atmosphere and the control cooling and heating sections of the furnace from ambient conditions. The function of the purge or transfer vestibule is to provide an inert atmosphere barrier so that laminations may be removed from the potentially explosive gas mixture without contact with air. Operating conditions for the purge section usually require that the static pressure be maintained at values slightly higher than the static pressure of the controlled cool section to minimize leaks of potentially explosive gas. A controlled oxidation section follows the purge vestibule. Here, a blue/gray oxide is formed on the surface of the annealed steel laminations. This oxide serves to provide a measure of electrical insulation for each of the laminations. The optimum temperatures known in the prior art for oxidation of steel to form a blue/gray reaction product range from above 600° to below 1000° F.
It has been known in the prior art cooling section to provide thin tubes containing a fluid such as water or air to cool the atmosphere by conduction into the fluid. Typically these have been arranged both above and below rollers of the transport conveyor, but at a substantial distance above and below the conveyor. Also it has been known to provide a re-circulation fan at the top of the chamber. These prior art structures are shown as the cooling water tubes 20 and 21, and recirculation fan 22 in FIG. 2.
In the prior art system, in order to increase production speeds, the heating section must be lengthened and the cooling section must be lengthened so that the resident time of the product in the heating section and the cooling section does not change as the conveyor is speeded up. This substantially increases cost both from a construction standpoint and from an energy standpoint.